The present invention relates to nuclear magnetic resonance imaging, sometimes called NMR. It is particularly applicable to uses in imaging body tissue in the medical field, since NMR is non-invasive and does not employ ionizing radiation.
Briefly, any nucleus with a magnetic moment tends to align itself in the direction of a magnetic field. However, the nucleus precesses around the magnetic field direction at a characteristic angular frequency which depends on the magnetic field strength and a constant relating to the particular nucleus properties. The components of the precession in the plane perpendicular to the magnetic field are in a random order. However, by applying a second magnetic field in the plane perpendicular to the first magnetic field, the precession will align to produce a net magnetic moment in that perpendicular plane. When the magnetic field is discontinued, the decay of the magnetic moment causes the emission of a radio frequency signal in the form of an oscillating sine wave. This radio frequency signal is measurable. By manipulating the magnetic field directions and gradients, data can be assembled to enable the generation of volumetric projections. A background description of nuclear magnetic resonance imaging and various methods for manipulating the magnetic fields can be found in U.S. Pat. Nos. 4,697,149 by Moran and U.S. Pat. No. 4,777,956 by Macovski and the references mentioned therein, all of which are hereby incorporated by reference.
The magnetic fields can be applied in a gradient along an axis to produce a frequency phase shift in the gradient direction. A series of gradients can be applied along one or more axes. The application of these field gradients allows moving material to be identified and the artifacts of motion to be desensitized. This can be done for a zeroth, first, second, third, and through jth order of motion to achieve rephasing of material that is static, material that has a constant velocity in the direction of the gradient, material that has a constant acceleration in the direction of the gradient, material that has a constant pulsatility in the direction of the gradient, and through material that has a jth order of motion in the direction of the gradient, respectively.
This method is described in U.S. Pat. No. 4,728,890 by Pattany et al., which is hereby incorporated by reference. In the preferred embodiment of that patent, the operator can select a zeroth, first, second, or third order motion desensitization. To make a zeroth order desensitization, i.e. rephase static material, a gradient pulse is added in each of the read and slice select gradient directions. The static or zeroth order gradient pulses are appropriately scaled in duration and amplitude such that the sum of the zeroth moments in time of the gradient pulses along each of the read and slice select directions is zero.
When a first order correction is selected, another gradient pulse is added to the pulse sequence such that the sum of the first moments in time along each of the slice select and read gradient directions are zero. Under the method of that patent, the sum of both the zeroth and first moments are set to zero to correct for both static rephasing and constant velocity artifacts. When a second order desensitization is selected, an additional gradient pulse is added such that the sum of the second moments in time of the gradients along each of the read and slice select directions is also zero. In this manner, correction is made for static components, constant velocity components, and acceleration. To correct for pulsatility related artifacts, a third order correction or desensitization is selected. In a third order correction, yet another gradient pulse is added such that the sums of the zeroth, first, second and third moments are set equal to zero. Analogously, fourth and higher order corrections may be made in a similar manner.
This approach is effective in desensitizing motion artifacts. However, in imaging moving materials such as in angiogram images, the goal is to distinguish the moving material from the static material. To do this, the static material must be dephased, preferably by a predetermined amount, while the moving material is rephased. Rephasing the moving material presents particular difficulties because the motion characteristics vary; that is, the velocity, acceleration, pulsatility and higher order motion components may vary from element to element.